The present invention relates to drill assemblies, and particularly to a venting and sealing systems used with drill assemblies having fluid-actuated pistons.
Drill assemblies, particularly down-hole drills, having fluid-actuated pistons are known, such as those disclosed in U.S. Pat. Nos. 5,085,284 of Fu, 5,301,761 of Fu et al., 5,562,170 of Wolfer et al., 5,711,205 (Wolfer et al.) and 5,566,771 of Wolfer et al. As shown in FIGS. 1 and 2, a typical down-hole drill assembly 1 includes a casing 2 containing the internal components of the drill assembly 1. A piston 4 is slidably mounted within the casing 2 and is guided by an inner bearing surface 2a of the casing 1 so as to reciprocally impact with a drill bit 5. The drill bit 5 provides the work output of the drill assembly 1. The piston 4 moves in either a drive direction, shown by arrow 3A, or a return direction, shown by arrow 3B. A fluid supply line 6 supplies high pressure or "percussive" fluid, preferably compressed air, to a supply chamber 7, the percussive fluid providing motive force for the piston 4 as discussed below.
The following description outlines both the basic structure and operation of the drill assembly 1. When the piston 4 is in close proximity to the bit 5 (FIG. 1), a return chamber 8 is in fluid communication with the supply chamber 7 via a return supply passage 9. Any pressure in the return chamber 7 biases the piston 4 in the return direction 3B. The fluid from the supply chamber 7 continues to be supplied to the return chamber 8 until a portion of the outer piston surface 4e passes across a sealing point 9a of the return supply passage 9. Fluid pressure in the return chamber 8 continues to accelerate the piston 4 in the return direction 3B until a lower end 4b of the piston 4 passes an outlet 5b in the bit 5. The outlet 5b leads into an exhaust chamber 11 formed within and extending through the bit 5, such that the pressurized fluid flows out of the return chamber 8 and into the chamber 11. However, the momentum of the piston 4 is such that the piston 4 continues moving in the return direction 3B.
At a certain point of the movement in the return direction 3B, the upper end 4a of the piston 4 engages the end 14b of an elongated guide portion 14 of a fluid distributor 15, which enters into and seals off a piston passage 4c (see upper portion of FIG. 2). After this point, percussive fluid in a drive chamber 13 above the piston 4 becomes compressed and increases in pressure as the volume of the chamber 13 decreases due to movement of the piston 4. The increasing drive chamber pressure decelerates movement of the piston 4 in the return direction 3B. Further, a pressure sensitive valve 16 regulates fluid flow through a distributor supply passage 17 that extends between the supply chamber 7 and the drive chamber 13. The distributor 15 also includes a distributor valve passage 20 and a distributor port 21 extending between the outer surface 14a of the guide portion 14 and the passage 20. Flow communication between a valve control chamber 19 (discussed below) and the drive chamber 13 is established through the distributor port 21 and the passage 20.
As best shown in FIG. 2, the valve 16 has three pressure surfaces, surfaces 16a, 16b and 16c. The first valve surface 16a is exposed to the pressure in the supply chamber 7, which tends to bias the valve 16 toward a valve seat portion 18 of the distributor 15 (i.e., to "close" the valve 16). When disposed adjacent the seat 18, the valve 16 obstructs the supply passage 17 and thereby prevents fluid communication between the supply chamber 7 and the drive chamber 13. The second valve surface 16b is exposed to pressure in the drive chamber 13 (through the supply passage 17), which tends to bias the valve 16 away from the valve seat 18 (i.e., to "open" the valve 16) and thereby establish flow communication between the supply chamber 7 and the drive chamber 13 through the distributor supply passage 17. The third valve surface 16c is exposed to pressure in the valve control chamber 19, which also tends to bias the valve 16 toward the valve seat 18.
As described above, movement of the piston 4 after engaging with the guide portion 14 causes the drive chamber pressure to increase. The increasing drive chamber pressure eventually causes the pressure acting on the second valve surface 16b to exceed the pressure acting on the first and third valve surfaces 16a, 16c, respectively. This pressure differential gives rise to a net force on the valve 16 that displaces the valve 16 from the valve seat 18 and thereby opens the distributor supply passage 17. Opening of the supply passage 17 enables high pressure percussive fluid to flow from the supply chamber 7 and into the drive chamber 13. The resulting pressure increase in the drive chamber 13 first halts the return travel of the piston 4, and then rapidly accelerates the piston 4 in the drive direction 3A.
As piston 4 travels in the drive direction 3A, the upper end 4a of the piston 4 passes the distributor port 21 such that pressurized fluid in the drive chamber 13 flows into the valve chamber 19 (via distributor port 21 and the distributor passage) to increase the pressure on the third valve surface 16c. Further, as the upper end 14a of the piston 14 passes the end 14b of the distributor guide portion 14, high pressure percussive fluid flows from the drive chamber 13 through the piston passage 4c and to the exhaust chamber 11. The resultant pressure decrease in the drive chamber 13, coupled with the pressure increase in the valve chamber 19, causes the valve 16 to be biased toward the valve seat 18 and thereby cut-off the flow of pressurized air from the supply chamber 7 to the drive chamber 13. The piston 4 then impacts with the bit 5 and the above-described cycle of movement of the piston 4 is repeated numerous times during operation of the drill assembly 1.
The operation of known drill assemblies, as discussed above, is adversely affected by inadequate control over the pressure in the valve control chamber 19. After the upper end 4a of the piston 4 passes over the distributor port 21, there should be no fluid communication between the drive chamber 13 and the valve chamber 19 as any increase in valve chamber pressure will prevent the valve 16 from opening in a timely manner. To prevent such fluid flow, the clearance between the interior surface 4d of the piston 4 and the outer surface 14a of the guide portion 14 must be negligible. Therefore, the piston interior surface 4d necessarily contacts and slides along the outer surface 14a of the guide portion 14, such that lubrication is required to minimize the adverse effects of metal-to-metal contact.
After a certain period of use of the drill assembly 1, wearing or galling of the piston interior surface 4d and the guide outer surface 14a inevitably occurs, such that the clearance increases. Thereafter, pressurized fluid from the drive chamber 13 flows or "leaks" between the surfaces 4d and 14a. The leakage flow causes a loss of pressure in the drive chamber 13, but more significantly, this flow enters the distributor port 21 and flows to the valve chamber 19. The resulting increase in valve chamber pressure increases the pressure acting on the third valve surface 16c, and thereby increases the minimum drive chamber pressure necessary to open the valve 16. Thus, as the percussive fluid in the drive chamber 13 must be compressed to a greater extent to achieve the increased pressure required, the valve 16 opens later in the piston movement cycle than desired.
One attempt to solve the above-described problem is to add a valve vent 12 to the fluid distributor 15. The valve vent 12 extends between the distributor passage 20 and an axial passage 22 through the distributor 15, the axial passage 22 being in fluid communication with the exhaust chamber 11 of the drill assembly 1. Excessive pressure in the valve control chamber 19 caused by fluid leaking between the piston interior surface 4d and the valve outer surface 14a is thereby directed through the valve vent 12 and to the exhaust chamber 11. The cross-sectional area of the valve vent 12 must be sufficiently large to enable the leakage flow from the drive chamber 13 to be vented sufficiently rapidly so that the valve chamber pressure does not increase.
However, the addition of the valve vent 12 to the fluid distributor 15 has been found to create a different problem. If the valve vent 12 is too large, percussive fluid that must be supplied to the valve chamber 19 during downward movement of the piston 4 (i.e., in the drive direction 3A) flows through the valve vent 12 instead of to the valve control chamber 19. The diversion of the fluid from the valve chamber 19, which is necessary to close the valve 16 when the piston 4 approaches the bit 5, prevents the valve 16 from closing at a desired point in the cycle of the piston movement.
In view of the above-discussed limitations with known down-hole drill assemblies 1 having fluid-actuated pistons, it would be desirable to have a venting and sealing system whereby the flow area for evacuating pressurized fluid from the valve chamber 19 to the exhaust chamber 11 was very large when the valve 16 must open (at or near the top of the stroke) and is zero or significantly small when the valve 16 must close (near the bottom of the stroke). It would also be desirable to significantly diminish, and preferably eliminate, the loss of pressurized fluid between the piston interior surface 4d and the outer surface 14a of the distributor guide portion 14 so as to improve the air consumption efficiency of the drill assembly 1. Further, it would also be desirable to provide a sealing system to reduce reliance on precision clearances between the piston 4 and the guide portion 14, such that the clearance therebetween is essentially negligible but the surfaces 4d and 14a were not prone to wear. Finally, it would be desirable to provide a system for sealing the space between the piston 4 and the distributor 15 which eliminated the need for oil or other lubrication to prevent metal-to-metal galling and wear, and thus permit lube-free operation of the drill assembly 1.